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Abstract:

Provided is a Ni-plated steel sheet being excellent in press formability
and suppression of wear of a mold and capable of maintaining alkali
battery characteristic even after a lapse of time. In a Ni-plated steel
sheet for a battery can having excellent press formability, an Fe--Ni
diffusion layer and a Ni layer formed on the Fe--Ni diffusion layer are
formed on a surface of a steel sheet corresponding to an outer surface of
the battery can, a semi-bright Ni plating layer is formed on the Ni
layer, a Ni coating weight of the Fe--Ni diffusion layer and the Ni layer
formed on the Fe--Ni diffusion layer is set equal to or smaller than a Ni
coating weight of the semi-bright Ni plating layer, the Ni coating weight
of the semi-bright Ni plating layer is 2.25 g/m2 or more, surface
roughness Ra1 of a surface of the semi-bright Ni plating layer in an area
of 2.5 μm×2.5 μm which is measured by an atomic force
microscope falls within a range from 3 to 11 nm, and surface roughness
Ra2 measured by a traceable roughness gauge is 0.3 μm or more and 2.0
μm or less with respect to the surface roughness Ra of the semi-bright
Ni plating layer.

Claims:

1. A Ni-plated steel sheet for a battery can having excellent press
formability, wherein an Fe--Ni diffusion layer and a Ni layer formed on
the Fe--Ni diffusion layer are formed on a surface of a steel sheet
corresponding to an outer surface of the battery can, a semi-bright Ni
plating layer is formed on the Ni layer, a Ni coating weight of the
Fe--Ni diffusion layer and the Ni layer formed on the Fe--Ni diffusion
layer is set equal to or smaller than a Ni coating weight of the
semi-bright Ni plating layer, the coating weight of the semi-bright Ni
plating layer is 2.25 g/m2 or more, surface roughness Ra1 of a
surface of the semi-bright Ni plating layer in an area of 2.5
μm×2.5 μm measured by an atomic force microscope falls within
a range from 3 to 11 nm, and surface roughness Rat of the surface of the
semi-bright Ni plating layer measured by a traceable roughness gauge is
0.3 μm or more and 2.0 μm or less.

2. The Ni-plated steel sheet for a battery can having excellent press
formability according to claim 1, wherein the Ni coating weight of the
Fe--Ni diffusion layer and the Ni layer formed on the Fe--Ni diffusion
layer is 1 to 5 g/m.sup.2.

Description:

BACKGROUND ART

[0001] Conventionally, a Ni-plated steel sheet has been popularly used as
a material for manufacturing battery cans. As properties which such a
Ni-plated steel sheet for manufacturing an alkali battery can is required
to possess, the stable press formability is required along with the
excellent battery characteristic and the excellent corrosion resistance.
As the stable press formability, a characteristic that the scratches are
not formed on a can at the time of can making, a characteristic that no
seizure is generated on a mold at the time of press forming or the like
is named.

[0002] Accordingly, as a characteristic of a material for manufacturing
cans, press formability is an important factor in terms of enhancing
productivity of cans by preventing a press machine from being shut down
due to the maintenance of a mold.

[0003] Further, in manufacturing an alkali battery can by press forming,
usually, it is considered unprofitable unless a mold can withstand the
manufacture of 700,000 to 1,000,000 cans (shots). Accordingly, from a
viewpoint of ensuring stable press formability, wear of a mold or
exchange frequency of a mold are regarded as important factors.

[0004] Further, from a viewpoint of environment, there has been a demand
for the reduction of an environmental load by performing degreasing of a
press product after press forming using an aqueous solution by promoting
the use of a non-organic solvent or non-alkali cleaning. Accordingly,
there has been a demand for a Ni-plated steel sheet which enables stable
press forming using a water-based emulsion or a press liquid of low
viscosity.

[0005] For example, following prior art documents can be named.

[0006] Patent document 1 (Japanese Patent 4051012) discloses a Ni-plated
steel sheet for a battery can where an Fe--Ni diffusion layer or an
Fe--Ni diffusion layer and a Ni-plated layer are provided to a surface of
a steel sheet which corresponds to an outer surface of a battery can, a
Ni-plated layer is further formed on the layer, and a surface roughness
Ra of the Ni-plated layer is set to 0.3 μm or more.

[0007] Patent document 2 (Japanese Patent 4051021) discloses a Ni-plated
steel sheet for manufacturing a battery can where an Fe--Ni diffusion
layer is formed on a surface of a steel sheet corresponding to an outer
surface of the battery can, and a Ni-plated layer is formed on the Fe--Ni
diffusion layer, surface roughness Ra of the Ni-plated layer is 0.1 μm
or more and 1 μm or less, and its Rmax is 1 μm or more and 10 μm
or less.

[0008] Further, Japanese Patent 3631143 (patent document 3) discloses that
the flow of batteries on a conveyor is excellently improved by applying
bright Ni--Co alloy plating to an outer surface of the battery can, and
there is also the description that the exchange frequency of a press mold
can be decreased by applying bright Ni--Co plating to an outer surface of
the battery can.

PATENT DOCUMENT

[0009] Patent document 1: Japanese Patent 4051012

[0010] Patent document 2: Japanese Patent 4051021

[0011] Patent document 3: Japanese Patent 3631143

SUMMARY OF THE INVENTION

Problems that the Invention is to Solve

[0012] There is no description relating to the exchange frequency of a
press mold in patent document 1 and document 2. That is, the Ni plated
steel sheets described in patent document 1 and patent document 2 have
drawbacks that stable press formability cannot be acquired, scratches are
generated on a battery can by can making, and severe seizure and wear
occur on a mold so that it is necessary to exchange the mold frequently.

[0013] The molded can disclosed in patent document 3 which is formed using
the Ni--Co alloy plated steel sheet can improve press formability and the
flow of batteries on a conveyor. However, contact resistance is increased
due to oxidation of a Ni--Co alloy plating layer so that there still
remains a drawback with respect to battery properties after a lapse of
time.

[0014] Accordingly, it is an object of the present invention to provide a
Ni-plated steel sheet for a battery can which can overcome the
above-mentioned drawbacks and has excellent property in press formability
and the suppression of wear of a mold.

[0015] Further, it is another object of the present invention to provide a
Ni-plated steel sheet for a battery can which can maintain excellent
alkali battery properties even after a lapse of time.

Means for Solving the Problems

[0016] (1) According to the present invention, there is provided a
Ni-plated steel sheet for a battery can having excellent press
formability, wherein

[0017] an Fe--Ni diffusion layer and a Ni layer formed on the Fe--Ni
diffusion layer are formed on a surface of a steel sheet corresponding to
an outer surface of the battery can,

[0018] a semi-bright Ni plating layer is formed on the Ni layer,

[0019] a Ni coating weight of the Fe--Ni diffusion layer and the Ni layer
formed on the Fe--Ni diffusion layer is set equal to or smaller than a Ni
coating weight of the semi-bright Ni plating layer,

[0020] a Ni coating weight of the semi-bright Ni plating layer is 2.25
g/m2 or more,

[0021] surface roughness Ra1 of a surface of the semi-bright Ni plating
layer in an area of 2.5 μm2.5 μm which is measured by an atomic
force microscope falls within a range from 3 to 11 nm, and

[0022] surface roughness Ra2 measured by a traceable roughness gauge is
0.3 μm or more and 2.0 μm or less.

[0023] (2) In the Ni-plated steel sheet for a battery can having excellent
press formability having the above-mentioned constitution (1), the Ni
coating weight of the Fe--Ni diffusion layer and the Ni layer formed on
the Fe--Ni diffusion layer is 1 to 5 g/m2.

Advantageous Effects of the Invention

[0024] The present invention can provide a Ni-plated steel sheet for a
battery can having the excellent press formability which is excellent in
the suppression of wear of a mold so that, in forming a battery can, the
generation of scratches on a formed can is suppressed and also seizure
hardly occurs on the mold.

MODE FOR CARRYING OUT THE INVENTION

[0025] An embodiment of the present invention is explained in detail
hereinafter.

[0026] <Definition>

[0027] Definition of Ra is described in JIS B0601-1994. Ra is an
arithmetic average height of roughness curve and an average value of
absolute value deviation from an average line. Ry is a maximum height and
indicates a height from the lowermost bottom part to the uppermost top
part for every reference length.

[0028] In the present invention, surface roughness Ra in an area of 2.5
μm×2.5 μm which is measured by an atomic force microscope is
expressed as surface roughness Ra1, and surface roughness Ra of the
surface in an area of 2.5 μm2.5 μm which is measured by a traceable
roughness gauge is described as surface roughness Ra2.

[0029] <Steel Sheet>

[0030] Usually, a low carbon aluminum-killed hot-rolled coil is used as a
substrate of a Ni-plated steel sheet for a battery can.

[0031] Further, a coil which is made of extremely-low carbon steel which
contains 0.003 weight % or less of carbon, or a coil which is made of
non-aging continuous cast steel manufactured by adding niobium and
titanium to the extremely-low carbon steel is used.

[0032] <Pretreatment before Plating>

[0033] As pretreatment performed before surface treatment, usually, a
scale (oxide film) formed on a surface of a cold-rolled steel sheet is
removed by applying electrolytic cleaning to a cold-rolled sheet using an
alkali solution containing caustic soda as a main agent or degreasing by
immersing a cold-rolled sheet in such an alkali solution. After the scale
is removed, the steel sheet is rolled to a product thickness in a
cold-rolling step.

[0034] <Annealing>

[0035] After rolling oil which adheres to the steel sheet in rolling is
cleaned by electrolytic cleaning, the steel sheet is annealed. Annealing
may be performed by either one of continuous annealing and box annealing,
and is not limited specifically. After annealing is applied to the steel
sheet, a shape of the steel sheet is modified.

[0036] <Ni Plating>

[0037] Next, Ni plating is applied to the steel sheet. In general,
although a nickel sulfate bath which is referred to as a watt bath is
mainly used as a Ni plating bath, besides the Ni sulfate bath, a sulfamic
acid bath, a fluoroborete bath, a chloride bath or the like can be used.
A Ni coating weight of Ni plating in performing plating using such a bath
is preferably set to 1 to 5 g/m2.

[0038] In a case where the Ni coating weight is less than 1 g/m2,
when heat diffusion treatment is performed, a Ni layer which is softened
(softened Ni layer) is not formed and the whole layer becomes the Fe--Ni
diffusion layer and hence, the steel sheet becomes disadvantageous in
terms of corrosion resistance.

[0039] On the other hand, the reason the Ni coating weight is set to 5
g/m2 or less is to prevent a thickness of the Fe--Ni diffusion layer
from becoming larger than necessary in an annealing step.

[0040] That is, there exists a tendency that when a Ni coating weight is
large, a thickness of the Fe--Ni diffusion layer is usually increased,
and the Fe--Ni diffusion layer is harder than the Ni layer or an iron
base material softened by annealing. Accordingly, there is a possibility
that when the Fe--Ni diffusion layer having a thickness larger than
necessity is formed, cracks are generated in the Fe--Ni diffusion layer
at the time of forming a battery can leading to the exposure of the iron
base material thus adversely influencing the corrosion resistance.
Accordingly, the Ni coating weight necessary for forming the Fe--Ni
diffusion layer is set to 1 to 2.25 g/m2.

[0041] Further, when the thickness of the Fe--Ni diffusion layer is large,
in press-forming a battery can, a heavy load is applied to the steel
sheet for working a thick and hard plated film and hence, a load is
applied to a mold thus giving rise to a possibility that wear on the mold
is induced.

[0042] <Ni-Plating Bath>

[0043] With respect to electrolysis conditions for acquiring a Ni plating
thickness, a case where a typical watt bath is used is explained. In this
case, the plating thickness can be acquired under the electrolysis
conditions where a bath has the bath composition containing 200 to 350
g/L of nickel sulfate, 20 to 50 g/L of nickel chloride and 20 to 50 g/L
of boric acid, has pH of 3.6 to 4.6 and has a bath temperature of 50 to
65° C., current density is set to 5 to 50 A/dm2 and the
number of coulomb is set to approximately 300 to 1500 c/dm2.

[0044] Here, besides matte Ni plating where an organic compound is not
added to a plating bath except for a pit prevention agent, semi-bright Ni
plating where an organic compound referred to as leveling agent which
makes a precipitated crystal surface of a plating layer smooth is added
to a plating bath, and bright Ni plating where an organic compound which
contains sulfur component for making a plating layer bright by making the
Ni plating crystal structure fine with the further addition of a leveling
agent is added to a plating bath are named. However, Ni plating which is
formed using a bath where an organic compound containing a sulfur
component is added to the bath is not preferable as Ni plating of the
present invention.

[0045] This is because, in thermal diffusion treatment which follows Ni
plating as a next step, the presence of the compound containing sulfur
causes brittleness thus deteriorating various properties such as
corrosion resistance.

[0046] <Formation of Diffusion Layer>

[0047] Next, heat treatment for forming an Fe--Ni diffusion layer is
performed after Ni plating. This heat treatment is provided for
increasing adhesiveness between base steel and a plating layer. The heat
treatment also forms an Fe--Ni diffusion layer, and a softened Ni plating
layer is left on the Fe--Ni diffusion layer.

[0048] As a method for diffusing Ni by heat treatment, a method which uses
a continuous annealing furnace or a method which uses a box-like
annealing furnace is named. In the method which uses the continuous
annealing furnace, a Ni diffusion temperature which falls within a range
from 600° C. to 700° C. and a Ni diffusion time which falls
within a range from 30 seconds to 120 seconds are used in usual Ni
diffusion by heating. An annealing atmosphere may be a non-oxidization
protective gas atmosphere or a reduction protective gas atmosphere.

[0049] When the heat treatment is performed at a temperature below
600° C. or for less than 30 seconds, the Ni plating which is
applied to the steel sheet does not form the Fe--Ni diffusion layer. On
the other hand, when the heat treatment is performed at a temperature
exceeding 700° C. or for more than 120 seconds, all Ni in the Ni
plating which is applied to the steel sheet diffuses into the base steel
thus forming the Fe--Ni diffusion layer and hence, the softened Ni layer
cannot be held on the diffusion layer. In the present invention, as a
heat treatment method by box annealing, it is possible to adopt the heat
treatment which uses a protective gas consisting of 75% of hydrogen and
25% of nitrogen produced by an ammonia crack method referred to as
hydrogen rich annealing with favorable heat transfer may be used. In this
method, favorable uniformity of temperature distribution is acquired in
the inside of a steel strip in the longitudinal direction as well as in
the widthwise direction of the steel strip and hence, the method has an
advantageous effect that the irregularities in the Fe--Ni diffusion layer
in the inside of the steel strip or the irregularities in the Fe--Ni
diffusion layer between steel strips can be decreased.

[0050] <Temper Rolling>

[0051] After diffusion treatment, the steel sheet is rolled by temper
rolling so that mechanical properties are imparted to the steel sheet,
and the surface roughness of a surface of the Ni plating layer which
forms a surface becoming an outer surface of a can is adjusted to a
predetermined roughness.

[0052] That is, the temper rolling is performed such that surface
roughness Ra2 measured by a traceable roughness gauge becomes 0.3 μm
or more and 2.0 μm or less.

[0053] This is because by adjusting the surface roughness Ra2 to a value
which falls within such a range, it is possible to set the surface
roughness Ra2 of semi-bright Ni plating (re-plating) which is a
succeeding step to a value which falls within a predetermined range.

[0054] <Semi-Bright Ni Plating (Re-Plating)>

[0055] After adjusting surface roughness to predetermined roughness by
temper rolling, semi-bright Ni plating of 2.25 g/m2 or more is
applied to the soft Ni layer being a surface which will become an outer
surface of a can.

[0056] When a coating weight of semi-bright Ni plating is less than 2.25
g/m2, a semi-bright Ni plating effect does not appear and hence,
sufficient press formability cannot be acquired. Further, there exists a
possibility that scratches are generated on a wall of a formed can or
seizure is generated on a mold.

[0057] Semi-bright Ni plating is characterized by forming a harder film
than matte Ni plating does. Further, semi-bright Ni plating is also
characterized in that oxidization advances slowly compared to bright Ni
plating and hence, the contact resistance is not increased thus
preventing semi-bright Ni plating from adversely influencing battery
characteristics. Accordingly, semi-bright Ni plating is suitable for
acquiring both press formability and battery characteristics by forming
semi-bright Ni plating on the soft Ni layer being a surface which will
become an outer surface of a can.

[0058] The reason a Ni coating weight of the Fe--Ni diffusion layer and
the softened Ni layer formed on the Fe--Ni diffusion layer is set equal
to or smaller than a Ni coating weight of the semi-bright Ni plating
layer is as follows. When the Ni coating weight of the Fe--Ni diffusion
layer softer than the semi-bright Ni plating layer and the softened Ni
layer formed on the Fe--Ni diffusion layer exceeds the Ni coating weight
of the semi-bright Ni plating layer, the semi-bright Ni plating layer
cannot acquire sufficient surface hardness so that wear resistance of the
semi-bright Ni plating layer against a mold which is an advantageous
effect of the semi-bright Ni plating layer cannot be acquired.

[0059] <Semi-Bright Ni Plating Bath>

[0060] As semi-bright Ni plating bath composition, the following watt bath
can be named.

[0071] Although the explanation is made later with respect to surface
roughness, to realize semi-bright Ni plating where surface roughness Ra1
of a surface of a semi-bright Ni plating layer in an area of 2.5
μm×2.5 μm which is measured by an atomic force microscope
falls within a range from 3 to 11 nm, it is important to control
concentration of semi-brightening agent and to make a value of pH fall
within the above-mentioned range.

[0072] Particularly, the concentration of the semi-brightening agent
largely influences surface roughness Ra1 of a plating film which is
measured by an atomic force microscope and contact resistance of the
semi-bright Ni plating film.

[0073] When a total amount of the semi-brightening agents 1, 2 is less
than 6.0 g/L, surface roughness Ra1 falls outside a specified range.

[0074] Further, when a total amount of the semi-brightening agents 1, 2
exceeds 12.0 g/L, the semi-bright Ni plating film is liable to be
oxidized leading to the lowering of a battery characteristic due to the
increased of contact resistance.

[0075] Further, to realize semi-bright Ni plating where surface roughness
Ra1 of a surface of a semi-bright Ni plating layer in an area of 2.5
μm×2.5 μm which is measured by an atomic force microscope
falls within a range from 3 to 11 nm, the following citric acid bath can
be also used.

[0076] Bath composition

[0077] nickel sulfate 6 hydrate: 300±50 g/L

[0078] nickel chloride 6 hydrate: 45±5 g/L

[0079] boric acid: 30±5
g/L

[0080] pit preventing agent: 2ml/L

[0081] pH: 4.5 to 5.0

[0082] bath
temperature: 65° C. or above

[0083] air agitation

[0084] Also with the use of citric acid bath, it is possible to acquire a
hard semi-bright Ni plating film where contact resistance is not
increased after a lapse of time.

[0085] <Surface Roughness Ra1 of Surface of Semi-Bright Ni Plating>

[0086] A surface of the semi-bright Ni plating is adjusted such that
surface roughness Ra1 of the surface of semi-bright Ni plating in an area
of 2.5 μm×2.5 μm which is measured by an atomic force
microscope falls within a range of 3 to 11 nm.

[0087] The resolution of an atomic force microscope is at a level which
enables the observation of atoms on a surface of a first layer, and
surface roughness Ra1 of a surface of a semi-bright Ni plating layer in
an area of 2.5 μm×2.5 μm which is measured by an atomic force
microscope depends on a grain size at the time of forming semi-bright Ni
plating. That is, the smaller the grain size of semi-bright Ni plating,
the smaller surface roughness Ra1 becomes. When the grain size of
semi-bright Ni plating is small, the semi-bright Ni plating film becomes
hard and hence, friction between the semi-bright Ni plating film and the
mold at the time of pressing can be made small. Accordingly, wear on the
mold is minimally generated so that the exchange frequency of the mold
can be reduced. That is, by setting surface roughness Ra1 to a small
value, press formability and mold wear suppression effect can be
enhanced.

[0088] In the present invention, by setting surface roughness Ra1 of a
surface of a semi-bright Ni plating layer in an area of 2.5
μm×2.5 μm which is measured by an atomic force microscope to
a value which falls within a range from 3 to 11 nm, it is possible to
acquire press formability and mold wear suppression effect.

[0089] <Surface Roughness Ra2 of Surface of Semi-Bright Ni Plating>

[0090] The resolution of a traceable roughness gauge is usually at a
several μm order, and surface roughness Ra2 of a surface of a
semi-bright Ni plating layer which is measured by a traceable roughness
gauge depends on roll roughness and a rolling load in a temper rolling
step. Although semi-bright Ni plating is applied after temper rolling in
this embodiment of the present invention, a grain size of a plating film
which is formed by applying semi-bright Ni plating has a nm order so that
such semi-bright Ni plating does not influence surface roughness Ra2. The
fact that the semi-bright Ni plating does not influence surface roughness
Ra2 is confirmed by measuring surface roughness Ra2 after temper rolling
is applied and surface roughness Ra2 after semi-bright Ni plating is
applied and by comparing these surface roughnesses Ra2 with each other.

[0091] In the temper rolling step, when the roll roughness is made large,
the roughness of the plated steel sheet to be rolled by temper rolling
becomes large, and when the rolling load is made large, the roughness of
an outermost surface of a plated steel sheet becomes large. By adjusting
the rolling roughness and the rolling load, the roughness of the
outermost surface of the plated steel sheet can be adjusted.

[0092] Rolls used for temper rolling are not specifically limited, and EDT
rolls can also be used besides shot dull rolls. That is, the rolls for
temper rolling are not specifically limited provided that a method where
the roughness falls within a range called for in Claims is adopted.
Further, a temper rolling facility is of a usual type and is not
specifically limited.

[0093] In the present invention, surface roughness Ra2 is set to a value
which falls within a range of 0.3 μm or more and 2.0 μm or less.

[0094] The reason is that in a case where surface roughness Ra2 is less
than 0.3 μm, when the mold comes in contact with a bottom surface of a
can at the time of pressing, the bottom surface of the can cannot
impregnate a minimum amount of lubricant so that scratches or seizure are
liable to be generated.

[0095] On the other hand, when surface roughness Ra2 exceeds 2.0 μm, an
amount of powder generated from the roll for tempering in the temper
rolling step is largely increased thus inducing quality defects such as
dents caused by powder particles. Accordingly, surface roughness Ra2
exceeding 2.0 μm is not preferable from a viewpoint of the
manufacture.

[0096] To prevent the generation of scratches and seizure, it is necessary
to impregnate a large amount of lubricant between a die of a mold and a
steel sheet. That is, a portion of a steel sheet which firstly comes into
contact with the die will become a bottom portion of a battery can, and
as the bottom portion is minimally worked, roughness of the steel sheet
remains. Accordingly, by setting surface roughness Ra2 after re-plating
to a value which falls within a range of 0.3 μm or more and 2.0 μm
or less, it is possible to apply an amount of lubricant which can prevent
the generation of scratches on a wall of a battery can or the generation
of seizure on a mold to a surface of the steel sheet.

[0097] <Measurement of Surface Roughness Ra2>

[0098] A measuring method of Ra2 is not specifically limited provided that
a roughness meter adopting a traceable surface roughness measuring method
is used. A measurement device is based on the description of
JIS-B0651-2001.

[0099] Although no other conditions are particularly specified with
respect to the measuring methods except for that measurement is performed
using the traceable surface roughness measurement device which is
calibrated using a reference sheet, the measurement is performed as
follows in this invention.

[0100] One example of measuring method is described hereinafter.

[0101] As the measurement device, a traceable surface roughness
measurement device (surfcom series) made by TOKYO SEIMITSU CO., LTD. is
used. JIS' 94 is adopted as the measuring condition. That is, the
measurement is performed under the conditions where an evaluation length:
5 mm, a measuring speed: 0.4 mm/sec, a cut-off value: 1.0 mm, a kind of a
filter: Gaussian, a measuring range: ±50 μm, gradient correction:
straight, and a cut-off ratio: 400.

[0102] <Press Forming>

[0103] A battery can is formed by press-forming a Ni-plated steel sheet.

[0104] Although a size of the battery can is not specifically limited, AA
and AAA which are popularly used in a digital camera or the like are main
sizes.

[0105] As a method of forming a battery can, a drawing and ironing method
by pressing is used, and the battery can is formed such that a thickness
of a can wall becomes equal from a bottom of the formed can (positive
pole terminal part in a form of the battery can) toward an opening part
excluding a calking portion of the battery can.

[0106] In the forming step, a 1st cup is formed by drawing a Ni-plated
steel sheet, the cup is subjected to drawing through six steps in total
and, thereafter, the cup is formed into a battery can through four steps.

[0107] As a material of the mold used at the time of press forming,
sintered hard alloy is preferably used. However, a kind of material of
the mold is not specifically limited. As a lubricant (press oil) used at
the time of press forming, two kinds of lubricants having low viscosity,
that is, a mineral-oil-based press oil and a water-soluble-emulsion-type
press oil can be named.

[0108] As the press oil having low viscosity, a press oil where kinematic
viscosity (40° C.) is 40 mm2/s or less is preferably used. It
is also possible to add a slight amount of additive such as molybdenum to
the press oil.

[0109] In a case where the press oil having low viscosity is used,
cleaning with a neutral surface active agent can be performed without
requiring an organic solvent or alkali cleaning in performing the
cleaning after forming a battery can and hence, the press oil works
extremely significantly in terms of an environmental load as well as a
cost. Accordingly, the use of the press oil is significant.

[0110] As the press oil having low viscosity, press oil shown in Table 1
can be favorably named.

[0111] As the water-soluble-emulsion-type press oil, in general, a
lubricant for preventing seizure can be used. The reason the
water-soluble-emulsion-type press oil is used as press oil is that such a
water-soluble-emulsion-type lubricant is water-soluble so that the use of
an organic solvent is unnecessary for cleaning after manufacturing a can
and hence, cleaning which takes into account an environment can be easily
performed. Accordingly, there is a possibility that the
water-soluble-emulsion-type press oil will be used widely in future.

[0112] Composition examples of water soluble emulsion press oil are named
hereinafter.

[0113] The above-mentioned composition is mixed at the above-mentioned
rates, and is further diluted with water until the concentration becomes
2 to 10% thus producing water-soluble emulsion lubricant.

EXAMPLE 1

[0114] A steel sheet having a sheet thickness of 0.25 mm is manufactured
using a low carbon aluminum-killed hot-rolled coil through cold rolling
and annealing, and a Ni plated steel sheet for a battery can according to
the present invention is prepared using the steel sheet as follows.
Matted Ni plating is applied to a surface of the steel sheet which will
become an outer surface of a can with a Ni coating thickness of 1.5
g/m2.

[0115] Further, matted Ni plating is applied to a surface of the steel
sheet which will become an inner surface of the can with a Ni coating
thickness of 10 g/m2. After applying Ni plating, continuous
annealing is performed at a temperature of 700° C. for 30 seconds
thus diffusing Ni in the Ni plating whereby an Fe--Ni diffusion layer and
the softened Ni layer on the Fe--Ni diffusion layer are formed.

[0116] A Ni coating weight of the Fe--Ni diffusion layer and the softened
Ni layer above the Fe--Ni diffusion layer is regarded as substantially
equal to a coating weight of a matted Ni plating layer before diffusion
treatment is applied.

[0117] After diffusion treatment is performed, temper rolling is performed
such that, with respect to surface roughness of a surface of the steel
sheet which will become an outer surface of a can, surface roughness Ra2
measured by a traceable roughness gauge becomes 0.3 μm.

[0119] After applying semi-bright Ni plating, surface roughness Ra1 of a
surface of the semi-bright Ni plating layer in an area of 2.5 μm2.5
μm within a measurement range of 10 μm square is measured by an
atomic force microscope (AFM), and the surface roughness Ra1 is 3 nm.

[0120] When the surface roughness of the steel sheet is again measured by
a traceable roughness gauge, it is confirmed that surface roughness Ra2
is 0.3 μm.

EXAMPLES 2 to 12

[0121] In the same manner as the example 1, Ni-plated steel sheets for a
battery can of examples 2 to 12 are prepared.

[0122] In the examples 2 to 12, however, matted Ni plating applied to a
surface of the Ni-plated steel sheet which becomes an outer surface of a
battery can, the surface roughness Ra2 (measured by the traceable
roughness gauge) of the surface of the steel sheet which will become the
outer surface of the can formed by temper rolling, the thickness of
re-plating applied to the soft Ni layer, and the surface roughness Ra1
measured by an atomic force microscope (AFM) after semi-bright Ni plating
are respectively changed as shown in Table 2.

COMPARISON EXAMPLES 1 to 6

[0123] Ni-plated steel sheets of comparison examples 1 to 6 are prepared
in such a manner that steel sheets substantially equal to the steel
sheets of the examples are used, and surface roughness Ra2 measured by a
traceable roughness gauge or a semi-bright Ni plating condition in
re-plating is set to a value which falls outside the range of the present
invention.

[0124] <Evaluation>

[0125] Table 2 shows a result of can making using the Ni plated steel
sheets of the examples and the comparison examples. In can making, the Ni
plated steel sheets of the examples and the comparison examples are
formed into battery cans for AA-size alkali batteries by a drawing and
ironing method using two kinds of lubricants consisting of
low-concentration mineral oil press oil and a water-soluble emulsion.

[0126] The battery can for a AA size alkali battery is formed through 10
steps. A material of a portion of a mold which is brought into contact
with the plated steel sheet is made of a WC-Ni material (NC-8) which uses
a Ni binder and, in a next step, a formed can having a can wall thickness
of 0.16 mmt which becomes a battery can for a U-3 size alkali battery is
obtained.

[0134] For evaluating the Ni plated steel sheets of the examples and the
comparison examples, seizure on a mold, the presence or the non-presence
of scratches on a formed can, and exchange frequency of the mold are
investigated. Further, 20 cans are extracted from the obtained battery
cans and alkali batteries are manufactured using these battery cans, and
the battery properties of these alkali batteries are evaluated.

[0135] The battery properties after preserving the alkali batteries at a
temperature of 60° C. for 20 days are obtained.

[0136] The evaluation of battery properties is carried out in accordance
with a photo flash mode of a typical use stipulated in an LR6 battery
evaluation item of ANSI, C18. 1M, part 1-2002.

[0137] "good" is given when the minimum number of times of flashes
described in ANSI is 210 or more and "bad" is given when the minimum
number of times of flashes described in ANSI is less than 210.

[0138] Specific discharge conditions are set to 1000 mA, 10 sec/min, and
frequency of 1 hr/day.

[0139] The Ni-plated steel sheets of the examples 1 to 12 which fall
within the scope of the present invention exhibit no "scratches on can"
and no "seizure on mold" as can be clearly understood from Table 2.
Accordingly, a mold can withstand pressing of 700,000 cans before being
exchanged due to wear of the mold generated by press forming. The
Ni-plated steel sheets of the examples 1 to 12 are proved excellent as
Ni-plated steel sheets for forming battery cans.

[0140] Further, battery cans which are manufactured using these Ni-plated
steel sheets of these examples also exhibit excellent battery properties.

[0141] On the other hand, the Ni-plated steel sheets of the comparison
examples 1 to 6 which fall outside the scope of the present invention
exhibit "scratches on a can" and "seizure on mold" so that the number of
cans which can be formed by molding is decreased and thereby the exchange
frequency of the mold is increased, and battery cans which are
manufactured using these Ni-plated steel sheets also exhibit deteriorated
battery properties.

[0142] According to the Ni-plated steel sheet for battery cans of the
present invention which has excellent press formability, it is possible
to provide a Ni-plated steel sheet where the generation of scratches at
the time of forming a battery can by a press forming is suppressed and
the seizure is minimally generated on a mold and hence, the present
invention possesses extremely high industrial applicability.